1. Field of the Invention
The present invention relates to a photopolymerizable type of dental resin composition which, when preparing full dentures or partial dentures for dentes defects, is provided to make base plates for bite plates used for the purpose of determining the relative positions of the upper and lower jaws, individual trays that are impression trays for individual patients to take final impressions and orthodontic resins to ameliorate malocclusion, restore cleft palate and cheiloschisis and ameliorate bottle mouths.
2. Prior Art
For the preparation of base plates for bite plates and individual trays, that are an impression tray for each individual patient, used so as to make full dentures or partial dentures for dentes defects as well as orthodontic resins, there has heretofore been used a powder/liquid type of cold-polymerizable resin in which polymethyl methacrylate and methyl methacrylate are mainly contained as the powder and liquid ingredients, respectively. The powder ingredient containing benzoyl peroxide as an initiator is mixed with the liquid ingredient containing a tertiary amine such as N,N'-dimethyl-p-toluidine as an accelerator, thereby initiating the reaction involved. Referring to the use of such resins, the required amounts of powder and liquid are weighed out and sufficiently mixed together in a mixing rubber cup with the use of a spatula. A resin paste, remaining admixed but not cured, is spread into a sheet form having a suitable thickness of, e.g., 1.5 to 2.5 mm for base plates or orthodontic resins and 1.0 to 5.0 mm for individual trays, and is then fitted under pressure onto a master cast of gypsum, to wait its curing. The orthodontic resin is used in combination with an orthodontic appliance--defined in a narrow sense--for the purpose of tooth movement, an orthopaedic appliance for the purpose of skeletal modification and a retaining appliance for the purpose of retention after minor tooth movement. In addition to the use of the resin in sheet forms, as mentioned above, the liquid may be applied direct onto the master cast of gypsum, followed by dusting of the powder. In this case, transparency is an important factor for the orthodontic resin and a pressure pot (about 2.5 to 3.0 kg/cm.sup.2) must be used to avoid the occurrence of air bubbles. Other materials such as thermoplastic resins or waxes may be used for base plates or individual trays. In recent years, materials for photopolymerizable base plates and individual trays in sheet forms have been developed. However, such materials are still far from perfection in terms of compatibility accuracy and maneuverability, since they use as the main ingredient monomer a liquid viscous at 40.degree. C. or lower.
Cold-polymerizable type resins such as resins for the above powder/liquid type of base plates or individual trays or orthodontic resins, materials for photopolymerizable base plates or individual trays in sheet forms in which a liquid viscous at 40.degree. C. or lower is used as the main ingredient monomer or materials for base plates or individual trays formed of thermoplastic resins or waxes have involved such problems as described below.
1. With the powder/liquid type of resin, it is required to weigh out the required amounts of powder and liquid, to admixing them together in a mixing rubber cup with the use of a spatula and to have the resulting resin paste in a sheet form. It is then impossible to keep the thickness of the sheet constant, because the sheet made pasty is a material so plastic that as soon as it is picked up by hand or fingers, it deforms and is thus partly thinned. Hence, it is likely that the base plates or individual trays or the orthodontic resin may break off due to stress concentrating upon the thinned portions.
2. The powder/liquid type of cold-polymerizable resin undergoes an exothermic curing reaction. Although a maximum temperature of exothermic heat varies depending upon the proportion of the powder and liquid to be admixed together, it reaches as high as about 60.degree. C. when 2 g of the powder are used per 1 ml of the liquid. Such heat generation results in thermal deformation which is so much increased that it produces an adverse influence upon the fitting of the base plate or orthodontic resin with respect to a master cast of gypsum.
3. The powder/liquid type of cold-polymerizable resin or the photopolymerizable sheet using as the main ingredient a viscous liquid at normal temperature shrinks considerably upon polymerization. The powder/liquid type of resin shows a linear shrinkage of 2 to 3% due to its combined heat shrinkage/shrinkage-upon-polymerization, whereas the photopolymerizable sheet using as the main ingredient a viscous liquid at 40.degree. C. or lower exhibits a linear shrinkage of 0.8 to 1.5% by shrinkage-upon-polymerization alone in the absence of substantial heat generation. This means that the compatibility of the base plate or orthodontic resin with respect to a master cast of gypsum is adversely affected.
4. The photopolymerizable sheet using as the main ingredient monomer a liquid viscous at 40.degree. C. or lower is very poor in maneuverability, since it becomes sticky on its surface when fitted under pressure onto a master cast of gypsum direct by hand or fingers. Moreover, the liquid viscous at 40.degree. C. or lower is liable to adhere to hands and fingers, possibly inducing eczema or an eruption on the skin.
5. When making an orthodontic resin with the powder/liquid type of cold-polymerizable resin, it has to be necessarily polymerized under pressure in a pressure pot so as to attain transparency by the removal of air bubbles. Since it is thus difficult to determine the time when the powder/liquid is to be made pasty and when the resin paste is to be pressurized in the pressure pot, some length of time and skill are required to obtain the orthodontic resin. Other materials for base plates or indiviudal trays formed of thermoplastic resins or waxes are also repeatedly softened by heat and fitted under pressure onto a master cast of gypsum to improve their compatibility. Hence, considerably longer length of time and skill are similarly required to for the accommodation in sophisticated cases such as in the case of partial dentures.
6. Materials for base plates or individual trays formed of thermoplastic resins or waxes have some strength in a thermally softened state, but have the property of deforming plastically or becoming very fragile when restored to normal temperature at which they are actually used. For the base plates, this implies that their strength is limited with respect to an occlusal pressure exerted during bite taking, so that they may break off or deform, thus making it impossible to prepare dentures of good compatibility. For the individual trays, this means that they may break off or deform, or make it impossible to achieve accurate impression taking. With the thermoplastic resins, neither is it possible to achieve myoplasty for which a compound softened at 70.degree. to 80.degree. C. is used and, hence, it is not possible to obtain any functional and precise master cast of gypsum. Thus, subsequent dental operations cannot be continued.